Added example of referring to AWS-credentials (#21560)

* Added example of referring to AWS-credentials

.. when they're stored in variables. Spent few hours trying to figure out why credentials from vault/variables were not used.

* Update guide_aws.rst

Edited for grammar.

docs/docsite/rst/guide_aws.rst

@@ -42,6 +42,13 @@ For storing these in a vars_file, ideally encrypted with ansible-vault::
     ec2_access_key: "--REMOVED--"
     ec2_secret_key: "--REMOVED--"
 
+Note that if you store your credentials in vars_file, you need to refer to them in each AWS-module. For example::
+
+    - ec2
+      aws_access_key: "{{ec2_access_key}}"
+      aws_secret_key: "{{ec2_secret_key}}"
+      image: "..."
+
 .. _aws_provisioning:
 
 Provisioning